Field of the Invention
The invention relates to a printing press including a plurality of printing or varnishing units and a dryer device for drying printed or varnished sheets.
Multi-color printing presses with additional varnishing unit(s) usually include a dryer device in which the layer of ink or varnish that has been applied is dried before the sheets are deposited on a sheet stack. It is not easy to dry the sheet sufficiently in the short time it remains in the dryer device, in particular if aqueous dispersion varnish is used. At the high machine speeds of 16,000 or 18,000 sheets per hour that are common today, the sheets frequently stay in the dryer for less than a second. One attempt to counteract this has been to manufacture machines with what is known as an “extended delivery” and to provide a plurality of dryer modules following each other in the delivery of the printing press, typically 3 to 4 dryer modules. Moreover, two or more drying towers are frequently used instead of just one drying tower for additional drying of the sheets between two varnishing units. Another attempt has been to increase the drying performance by increasing the temperature and/or the hot air stream. For instance, in machines of the 70/100 format, it is not uncommon today to use a dryer device with a wattage of more than 100 kW in the delivery and even 125 kW in the drying towers between the varnishing units and to extend the dryer section in the delivery to 4 m.
However, increasing the wattage of the dryer devices does not make sense in economic terms because a large proportion of the energy is not used to evaporate the solvents from the ink or the water from the layer of varnish, but is lost as waste heat. On one hand, this is due to the fact that the degasification or evaporation process does not start at full strength when the sheet enters the dryer because a large proportion of the energy is required initially to heat up the sheet of paper itself. On the other hand, the measure of extending the delivery and adding dryer units considerably increases the installation space required for such a printing press. Apart from the fact that such a measure becomes impossible in some cases for reasons of space limitations, an additional dryer unit or an extended delivery means additional costs that increase the price of the printing press.
It is known from International Patent Application No. WO 01/68223 A1, corresponding to U.S. Pat. No. 6,868,788, to use the waste heat of the printing press for pre-heating the air of a thermal dryer. However, that measure is insufficient to solve the problems indicated above, i.e. the high energy consumption of the dryer itself and the long dryer sections.
Another option is the introduction of what are known as “interdeck dryers” into every printing unit in question to dry the ink or varnish directly at that location. Apart from the fact that such a measure is highly complex, such interdeck dryers have their drawbacks inasmuch as the sheets are heated up during an ongoing printing operation. That may have a negative effect on the quality of the printed image because the conditions in the press during a printing operation are desired to be as uniform as possible. As a consequence, additional steps must be taken, for example to compensate for an uneven heating up of an impression cylinder equipped with an interdeck dryer, as described in German Published, Non-Prosecuted Patent Application DE 10 2005 022 595 A1. Alternatively, cooled blown air is introduced into the path of sheet travel to cool down the sheet in the subsequent printing unit to the desired temperature for the printing operation, as described in European Patent Application EP 1 502 738 A1, corresponding to U.S. Pat. No. 7,044,059.
For those reasons, interdeck dryers are usually not thermal dryers, but mainly UV dryers used in connection with printing inks that harden under the influence of ultraviolet light.
Furthermore, German Published, Non-Prosecuted Patent Application DE 103 05 594 A1 discloses an interdeck dryer wherein the impression cylinder of a printing or varnishing unit is heated up.
European Patent EP 1 287 204 B1, corresponding to U.S. Pat. No. 7,017,493, describes heated smoothing rollers for treating the printed or varnished sheet surface and for heating up the printed sheet. However, that document also describes additional cooling rollers for cooling down the printing material or printed sheet after the smoothing operation. The problems of the dryer that have been described above are not dealt with in that document.